Various types of acoustic sensing are used during hydrocarbon production. Generally speaking, acoustic sensing may be active or passive. Active acoustic sensing applications include vertical seismic profiling and hydrofracture monitoring, or any other application that includes source activation. In hydrofracture monitoring, for example, a fluid may be injected into the formation to stimulate microseismic activity. Passive applications do not use ancillary means to stimulate the well but instead simply monitor acoustic activity created during production, or monitoring. In each sensing application, the acoustic sensors are used to obtain information to help operators better understand the characteristics of the fracture, such as propagation direction, geometry, dip, and other parameters. In active monitoring applications, the information permits adjustment, in real time, of the injection process parameters in case of a mismatch between the planned and actual fracture. The “real-time” expression refers here to enabling a reaction in a time which is fast enough to have an impact on the economics of the fracturing job (for instance stopping the injection in case the fracture is in danger of extending into a water zone).
Many current microseismic fracture monitoring systems employ a wireline seismic array tool deployed in a monitoring well. A monitoring well, however, is not always available or suitable for microseismic monitoring (e.g., if located too far from the injection well). In addition, there is a high cost associated with running a seismic wireline tool in a monitoring well, mainly coming from the preparation of the monitoring well.
Other fracture monitoring systems have been proposed in which the sensors are deployed in the injection well, thereby eliminating the need for a monitoring well. These systems, however, are typically limited to use in applications where tubing is used to convey the fracture fluid.